1. Field of the Invention
The present invention relates generally to wideband power amplification, and in particular, to a predistorter for linearly amplifying a wideband Radio Frequency (RF) signal and a method of adapting the predistorter.
2. Description of the Related Art
RF amplifiers are used in a typical mobile communication system and communicate via RF signals. The RF amplifiers are divided into low-power, low-noise receive amplifiers and high-power transmit amplifiers. Efficiency is a more significant consideration than noise in a high-power transmit amplifier. A high-power amplifier (HPA), widely used to achieve high efficiency in mobile communication applications, operates near a non-linear operation point.
Intermodulation distortion (IMD) from the amplifier output adversely affects out of band frequencies as well as in band frequencies with spurious signals. A feed forward method is usually adopted to eliminate the spurious component. Despite the advantage of complete elimination of the spurious component, however, the feed forward method has a low amplification efficiency and requires control at an RF stage. Therefore, the HPA becomes bulky and system cost is increased.
Digital predistortion (DPD) is being studied as a means to achieve high efficiency and low cost in the mobile communication industry. The DPD preliminarily compensates for the losses of an input signal by adding an inverse of the non-linearity of a non-linear amplifier and renders the output of the non-linear amplifier linear. The non-linearity of the non-linear amplifier is represented as Amplitude Modulation to Amplitude Modulation (AM to AM) conversion distortion and Amplitude Modulation to Phase Modulation (AM to PM) conversion distortion. The AM to AM conversion distortion is defined as a change in the amplitude of an output signal with the amplitude of an input signal, while the AM to PM conversion distortion is defined as a change in the phase of the output signal with the amplitude of the input signal.
Most predistorters work for single tone signals or narrow band frequency signals. Therefore, they generally compensate for the memoryless non-linearity of a non-linear amplifier without considering memory effects. The memoryless non-linearity refers to the present output being influenced by the present input only. However, the memoryless non-linearity of the non-linear amplifier at a wideband frequency causes previous input signals as well as the present input signal to affect the present amplifier output, thereby substantially changing the AM to AM and AM to PM characteristics. This phenomenon is called memory effects. The non-linearity of a power amplifier varies with the frequency of an input signal.
The increasing use of wideband frequencies in mobile communication systems has driven research and development toward the memoryless effects of non-linear amplifies. The main techniques of compensating for both the memoryless non-linearity and memory effects of a non-linear amplifier are a simplified Volterra model, a memory polynomial model, and Hammerstein predistortion.
A Volterra series can be seen as a Taylor series with a memory. The Volterra series is used to accurately model a non-linear system. Thus, a Volterra model eliminates the non-linearity of a non-linear amplifier by an inverse of a Volterra series model that simulates the non-linear amplifier. This scheme is effective in completely eliminating the non-linearity of the non-linear amplifier, but its physical implementation is almost impossible because of the required use of a complicated formula for high-order inversion and the requirement of a large volume of computations.
The memory polynomial model is a simplified version of the Volterra model. It represents an output as a polynomial of a present input and previous inputs. The ability to eliminate the non-linearity of an amplifier depends on the number of previous inputs considered and the order of the polynomial. An additional polynomial is added each time an additional previous input is considered. Therefore, the memory polynomial model is likely to increase the volume of required computation.
The Hammerstein predistortion is a serial connection of a memoryless non-linear system and a linear time invariant (LTI) system. A traditional Hammerstein model calculates accurate coefficients of the memoryless non-linear system and the LTI system by matrix inversion. The matrix inversion is a calculation of coefficients by arranging equations satisfying some conditions in the form of a matrix and finding the inverse of the matrix. This scheme also requires too many multiplications for inversion, which makes it impossible to physically implement the Hammerstein predistortion.